One of the main things you learn in school is how to learn. It's a process. You're learning process, and how to reach B from A via C.
Teachers are looking for the steps to make sure that you understood the process. Jumping from A to B doesn't show that the process is understood.
He has a quick mind, this is good. I used to be one of those kids who could come up with the answers in my head as fast or faster than via paper. I would just "see" the answer. It's not as good as it used to be, but I understand how he can be frustrated.
One lesson I had to learn is "In life, sometimes it's not about winning (or getting the right answer) it's about playing the "game"". In this case, the "game" is the math exercise, and the game's rules have been determined by the teacher. Most of us can win a game that we make the rules to. The challenge comes when we don't make the rules.
It's character building. Your boy seems to have a lot of character already, but we can all use a little more. That's what some of these sewing machines continually insist on teaching me anyway.